Instruments for measuring the reflectance of reagent test devices or reagent strips are well known. Such instruments typically measure the reflectance from a reagent pad of the test device or reagent strip in order to determine the presence and/or the amount of analytes, such as glucose, protein, ketone and the like, in body fluids. The reagent pad is a reagent impregnated area designed to react with a component (analyte) of a body fluid to provide a detectable change such as a color change. A color change can be measured by reflected light to provide a qualitative and/or quantitative indication of a particular analyte in the body fluid being tested.
Ambient light, i.e., light exterior to reflectance measuring instruments, can cause interference with the signal light of such instruments. Consequently, reflectance measurements are normally made inside reflectance measuring instruments where ambient light is blocked. Typically reflectance measuring instruments have an opaque door which is opened to allow placement of a test device or test strip. The door is then closed to block ambient light from the area where the reflectance measurement is made.
When a readhead door on a commercially available reflectance photometer is left open the measured reflectance can increase by at least 10% due to ambient light interference with the signal light. The affect of this is shown in FIG. 1 where a reflectance photometer is measuring glucose concentration ([G] in units of of milligrams per deciliter [mg/dL] of whole blood) as a function of measured reflectance. For example, when the reflectance changes from 0.250 reflectance units to 0.275 reflectance units (an increase of +10%) the reported result to the user changes from 116.1 to 89.5 mg/dL, which is a change of 23%. The required clinical accuracy of the whole system, including instrument, reagent pad, etc. must typically be less than 15%. It can be seen that a mere 10% increase in detected light level can result in inaccuracies of 23%, an amount which exceeds the clinical accuracy required for reflectance measuring instruments. It should also be noted that ambient light has more of an affect on [G] at different reflectance levels. This is particularly true at higher glucose readings.
In U.S. Pat. No. Des. 245,434 a reagent tray designed to hold a reagent test device is employed to transport a reagent test device into a reflectance photometer where reflectance measurements are made in an area substantially unaffected by ambient light.
In U.S. Pat. No. Des. 292,277, on the other hand, a more typical reflectance photometer is illustrated having a readhead door which is opened and closed. The readhead door is opened to insert a reagent test device and then closed prior to making a reflectance measurement. The closed door substantially prevents ambient light from interfering with the reflectance measurement. In order to remove the reagent test device from such a reflectance photometer it is necessary to again open the readhead door.
Notwithstanding the effectiveness of these approaches, neither of these prior procedures is attractive from the standpoint of user convenience, engineering simplicity and/or manufacturing costs. Customer convenience requires simple operating sequences with as few steps as possible. Another problem with the prior art procedures for suppressing or blocking ambient light is the fact that it is sometimes difficult to correctly position a reagent test device in an instrument because the final position of the reagent pad on the test device cannot be precisely determined when the reagent pad is inside the instrument upside down (facing the readhead window) and the door or lid to the reflectance measuring instrument is closed.
Accordingly, there is a need for means which simplify the positioning of a reagent test device or test strip in a reflectance photometer while simultaneously suppressing ambient light which will alter any measurement from its actual value. The present invention achieves these goals, eliminating the necessity for complex tray mechanisms to ratchet a reagent test device into a reflectance photometer for measurement and/or using a door or lid mechanism to suppress or prevent ambient light from interfering with reflectance measurements.